The present invention relates to an adaptive receiver that acquires and tracks the carrier frequency and phase of QAM and MPSK signals under fading channel conditions, and provides reliable data estimates under such fading conditions encountered in wireless communication. The adaptive receiver utilizes an adaptive filter, a phase detector, a Kalman filter, a fixed lag smoother, and a smoothed symbol detector to provide an accurate estimate of the channel fade envelope, to compensate for the phase dynamics induced due to time-varying atmospheric or terrestrial multipath fading, and to provide reliable estimates of the symbols of high-order modulated signals.
Wireless communication systems are currently in a rapid evolutionary phase in terms of development of various technologies, development of various applications, deployment of various services, and generation of many important standards in the field. Although there are many factors to be considered in the design of these systems, important factors include the bandwidth utilization efficiency due to the limited bandwidth allocation, flexibility in operation and robustness of the communication link in the presence of various disturbances such as fading while achieving the specified performance.
The bandwidth and power efficient communication systems require coherent multilevel modulation techniques such as MQAM with M equal to 16 or higher, and MPSK signals. While both MQAM and MPSK signals have the same bandwidth efficiency for any value of M, the MQAM signals have a much higher power efficiency compared to MPSK. However, the detection of MQAM signals for M higher than 4 requires an accurate knowledge of the signal amplitude which is relatively difficult to obtain under time-varying random variations in the channel gain as occurs, for example, in Rayleigh fading channels in wireless communication. In the absence of a technique for a fast and accurate tracking of the fade envelope of the channel, the application of the MQAM techniques for M greater than 4 has serious limitations in terms of their applications to wireless channels.
Additionally, both MQAM and MPSK techniques require an accurate estimation of the carrier frequency and phase which are corrupted by the phase noise introduced by the fading channel, in addition to any oscillator noise that is present in non fading channels. However, the magnitude of the phase noise due to the channel is much higher compared to that due to the oscillator. The required accuracy of the carrier phase increases with order M for both the MQAM and MPSK signals. Thus, successful applications of high order QAM techniques to wireless fading communication channels require both an accurate carrier phase acquisition and tracking, and a fast and accurate tracking of the channel fade envelope.
In order to achieve high power efficiency, the carrier reference is obtained by some processing of the data modulated signal itself rather than transmitting a pilot carrier signal which results in power loss. Among such techniques is the loop involving a fourth power circuit followed by a narrow band phase lock loop that tracks four times the carrier frequency. This is suitable for QPSK modulation, which is the same as MQAM or MPSK with M equal to 4. See “Using Times-Four Carrier Recovery in M-QAM Digital Radio Receivers,” by A. J. Rustako, et. al., IEEE Journal on Selected Areas in Communications, vol. SAC-5, No. 3, pp. 524-533, April 1987. In a fading environment, a limiter precedes the fourth power circuit to eliminate the gain variations in the loop. See “A Limiter Aided 4th Multiplying PLL Carrier Recovery Technique for 16-QAM Signal,” IEEE, 1997. However, the process of limiting amplifies the noise. Also, the method is vulnerable to phase jitter induced by random data patterns. Such techniques involving limiting and taking power M of the signal are also applicable to higher order MPSK signals, albeit with the same disadvantages and do not extend to MQAM with M>4. Decision-directed methods known as polarity-type Costas loop for the MPSK signals involve slicing the inphase and quadrature components of the received signal. See,” A Generalized Polarity-Type Costas Loop for Tracking MPSK Signals,” IEEE Transactions on Communications, Vol. 30, N0. 10, pp. 2289-2296, October 1982. This requires the knowledge of the signal amplitude and thus does not apply to MQAM signals with M>4 under fading conditions. In the absence of accurate channel gain estimate in the fading environment, the application of the decision-directed methods to MQAM is not feasible and results in significant degradation in performance in the data estimates along with frequent loss of lock and long acquisition times. An earlier proposed solution to this problem by Dobrica, Carrier Synchronization Unit, U.S. Pat. No. 5,875,215, Feb. 23, 1999, used pilot symbols to estimate the channel gain. However, the use of pilot symbols result in significant reduction in capacity of the channel and requires interpolation of the amplitude and phase estimates from the pilot symbols to the subsequent data symbols. This leads to significant errors in amplitude and phase estimates during the data symbol detection, and imposes logistic difficulties in terms of maintaining the pilot symbol sequences, the need for frame synchronization, etc. Kumar, U.S. Pat. No. 6,693,979, Feb. 17, 2004 teaches a receiver for improved phase estimation using fixed-lag smoothing by estimating the fading channel amplitude. However, in the earlier teaching of Kumar there is no data modulation considered and thus does not solve the problem of reliable detection of high-order modulated signal over fading channels without the need for any pilot signals. These and other disadvantages are solved or reduced by the receiver of the present invention.